1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) and a method for manufacturing the same, and more particularly, to a method for manufacturing a substrate for an LCD having thin film transistors (TFT), and a method for manufacturing the same.
2. Discussion of the Related Art
The structure of a conventional LCD will be described with reference to FIG. 1. The LCD has a first substrate 3 on which pixels are formed in a matrix array. On the first substrate 3, pixel electrodes 4 are formed and each pixel electrode 4 is surrounded by a gate bus line 17 and a data bus line 15. A gate electrode 17a branches off from the gate bus lines 17 and a source electrode 15a branches off from the data bus line 15. A TFT 8 is formed at the intersection of the gate bus line 17 and the data bus line 15. A drain electrode 15b of the TFT is formed to make an electrical contact with the pixel electrode 4. A black matrix (light shielding layer) is formed to cover TFTs 8, gate bus lines 17, and data bus lines 15. An alignment film (alignment layer) is formed on the overall surface of the substrate including the black matrix.
A second substrate 2 having a color filter layer 37 is prepared to face the first substrate 3, defining a gap between the two substrates. A liquid crystal material 40 fills the gap. On the outer sides of the first and second substrates, polarizing plates 1 and 1a are attached. This completes the conventional LCD panel.
Referring to FIGS. 2A and 2B, the structure of the first substrate 3 in the conventional LCD will be described in detail. FIGS. 2A and 2B are cross-sectional views taken along line I-I' of FIG. 1. A process for forming the structure of the first substrate and its individual components are explained with reference to FIG. 2A.
A gate electrode 17a branching off from a gate bus line 17 is formed on a transparent substrate 3. An anodized film 35 is formed on the gate electrode 17a to improve the insulation quality and prevent hillocks. A gate insulation film 23, which is made of an inorganic material such as SiN.sub.x or SiO.sub.2, is formed on the overall surface including the gate electrode 17a. A semiconductor layer 22 of amorphous silicon (or a-Si) is formed on the gate insulation film 23 over the gate electrode 17a. This is followed by the formation of an impurity-doped semiconductor layer 25 such as impurity-doped amorphous silicon (N.sup.+ a-Si). On the impurity-doped semiconductor layer 25, a source electrode 15a branching off from a data bus line 15 and a drain electrode 15b are formed with a certain gap between them. Here, the source electrode 15a and the drain electrode 15b make ohmic contacts with the impurity-doped semiconductor layer 25. An inorganic passivation (protection) film 26, such as SiN.sub.x, is formed to cover the entire surface including the source electrode 15a and the drain electrode 15b. A pixel electrode 4 is formed on the passivation film 26 so as to make an electrical contact with the drain electrode 15b through a contact hole formed in the passivation film 26 located over the drain electrode 15b. Then, a black matrix 10 is formed such that it covers the TFT 8, the gate bus line 17, and the data bus line 15 (FIGS. 3 and 4). This is followed by coating an alignment film 11 made of polyimide, for example.
Another possible structure of the first substrate 3 in the conventional LCD is illustrated in FIG. 2B, which shows components similar to those in FIG. 2A. In this example, an alignment film 11 is formed prior to the formation of the black matrix 10 to prevent an improper rubbing problem of the alignment film 11 near the black matrix 10.
Moreover, the LCD having the structure of FIG. 2A or 2B has the following problems. First, in the structure of the first substrate shown in FIG. 2A, the alignment film has a stepped profile due to steps formed by the pixel electrode 4 and black matrix 10. This results in improper rubbing of the alignment film near the steps, which in turn causes light leakage. Thus, the quality of an LCD such as contrast is reduced. To better understand this phenomenon, the formation and rubbing of an alignment film are explained in detail with reference to FIGS. 5 and 6.
FIG. 5 is a cross-sectional view taken along line III-III' in FIG. 4. The alignment film 11 in FIG. 2A is formed by transferring a material for alignment film, such as polyamide, polyimide, or silicon oxide, printed on a roller 150 onto the surface of the first substrate 3 including the black matrix 10. The alignment film is then hardened and rubbed to align liquid crystal in a direction. As shown in FIG. 6, the rubbing process creates grooves (wave-like lines in the figure) on the alignment film 11 in a certain direction by using a rubbing drum 131. Here, the rubbing drum 131 is lapped with a rubbing cloth 130 and moved in the C direction while being energized in the B direction and rotating in the A direction. A portion D.sub.0 marked by oblique lines in FIG. 4 (133 in FIG. 6) represents the area where the rubbing process is not properly carried out due to the steps formed by the black matrix 10. The width of D.sub.0 in FIG. 4 is 1-2 .mu.m when the thickness of black matrix is 1-2.5 .mu.m. Such a region may be eliminated by a photo-array of an alignment film using polyvinylcinnamate (PVCN), polyvinylfluorocinnamate (PVCN-F), polysiloxanes, or polyvinylchloride (PVC). However, the problem of a non-uniform cell gap, which will be described next, still remains unsolved.
Second, as shown in FIGS. 2A and 2B, the first substrate 3 in the conventional LCD reveals a stepped surface due to a multi-layer structure including a black matrix 10. This causes a non-uniform cell gap for the LCD. Thus, the quality of the LCD and the yield decrease due to instability in filling liquid crystal in the gap. In addition, the quality of an LCD cannot be maintained if the liquid crystal is in direct contact with the black matrix as shown in FIG. 2B. The black matrix or its color pigment may contaminate the liquid crystal. In general, a black matrix is made of a negative photoresist containing black color pigment.
Third, in the conventional LCD, pixel electrodes cannot be formed to overlap data or gate bus lines. This is due to the stepped surface of the first substrate, the improper rubbing of the alignment film, and a high dielectric constant of an inorganic insulation film (protection film 26). If a pixel electrode is formed to overlap a data bus line located beneath an inorganic insulation film, blinking or flickering of a display occurs because of the interference between the data bus line voltage and the pixel electrode voltage. Light leakage due to the improper rubbing may also arise at the overlapping portion. Thus, in general, the pixel electrode is disposed a certain distance away from the step of data bus lines. In this case, a sufficient aperture ratio cannot be obtained as illustrated in FIG. 7. In FIG. 7, an inorganic insulation film 26 such as SiN.sub.x or SiO.sub.2 covers the stepped data bus-line 15, and a pixel electrode 4 is formed a distance D.sub.2 away from the data bus line 15. The distance D.sub.1 provides the tolerance in assembling the first substrate and the second substrate. Thus, the aperture ratio in the conventional LCD is smaller than the maximum possible value by the area determined by D.sub.3 =(D.sub.1 +D.sub.2).
As explained in detail above, the first substrate has a stepped surface due to a black matrix and/or the black matrix is in direct contact with a liquid crystal material. This results in problems, such as light leakage through the area near the steps of the black matrix, a non-uniform cell gap, contamination of liquid crystal, and a small aperture ratio.